WASHINGTON (Reuters) - U.S. auto safety regulators are examining welding as part of their investigation into battery fires in Chevy Volts that could lead to a recall for the plug-in car.

The National Highway Traffic Safety Administration wants to know whether there are any visible weld changes at key points on the underside of the vehicle made by General Motors near the battery pack, according to an investigative memo that was publicly disclosed.

The memo gave no indication of whether the agency would push for a recall.

Investigators plan to analyze photographs of the floor pan and cross member structural support at the site where the 400-pound lithium-ion battery was damaged in a rigorous side crash test last May.

NHTSA requested that its test center complete the work ”at the “earliest opportunity.”

The agency owns five Volts purchased for crash testing and other safety evaluation.

The request was an early indication of NHTSA thinking in its preliminary examination of why Volt batteries ignited following government testing after the spring crash trials and in follow-up battery tests weeks ago that prompted the formal investigation.

The memo from NHTSA headquarters to its testing center raised no questions about the structural integrity of the vehicle.

GM is also trying to identify the problem and devise a remedy.

Reinforcing the case surrounding the battery is one proposed remedy GM is considering, sources with knowledge of the matter told Reuters on Tuesday.

GM has placed a huge bet on the Volt as a leader in fuel economy and green technology with the enthusiastic backing of the Obama administration, which says the car is safe to drive.

NHTSA has given the Volt a top “five-star” safety rating, but notes that it is the subject of an ongoing investigation on its consumer website.